The present invention relates to the separation of gas into components, using polymeric membranes.
It has been known to use a polymeric membrane to separate air into components. Various polymers have the property that they allow different gases to flow through, or permeate, the membrane, at different rates. A polymer used in air separation, for example, will pass oxygen and nitrogen at different rates. The gas that preferentially flows through the membrane wall is called the “permeate” gas, and the gas that tends not to flow through the membrane is called the “non-permeate” or “retentate” gas. The selectivity of the membrane is a measure of the degree to which the membrane allows one component, but not the other, to pass through.
A membrane-based gas separation system has the inherent advantage that the system does not require the transportation, storage, and handling of cryogenic liquids. Also, a membrane system requires relatively little energy. The membrane itself has no moving parts; the only moving part in the overall membrane system is usually the compressor which provides the gas to be fed to the membrane.
A gas separation membrane unit is typically provided in the form of a module containing a large number of small, hollow fibers made of the selected polymeric membrane material. The module is generally cylindrical, and terminates in a pair of tubesheets which anchor the hollow fibers. The tubesheets are impervious to gas. The fibers are mounted so as to extend through the tubesheets, so that gas flowing through the interior of the fibers (known in the art as the bore side) can effectively bypass the tubesheets. But gas flowing in the region external to the fibers (known as the shell side) cannot pass through the tubesheets.
In operation, a gas is introduced into a membrane module, the gas being directed to flow through the bore side of the fibers. One component of the gas permeates through the fiber walls, and emerges on the shell side of the fibers, while the other, non-permeate, component tends to flow straight through the bores of the fibers. The non-permeate component comprises a product stream that emerges from the bore sides of the fibers at the outlet end of the module.
Alternatively, the gas can be introduced from the shell side of the module. In this case, the permeate is withdrawn from the bore side, and the non-permeate is taken from the shell side.
An example of a membrane-based air separation system is given in U.S. Pat. No. 4,881,953, the disclosure of which is incorporated by reference herein.
Other examples of fiber membrane modules are given in U.S. Pat. Nos. 7,497,894, 7,517,388, 7,578,871, and 7,662,333, the disclosures of which are all hereby incorporated by reference.
Two major criteria used in designing a membrane-based gas separation system are 1) the selectivity of the membrane, and 2) the amount of energy required to operate the compressor. In many applications, one desires gaseous product streams of high purity, while minimizing the amount of energy used in operating the compressor.
The flow rate of a gaseous component through a membrane is proportional to the difference in partial pressures of that component, on either side of the membrane. Therefore, a membrane which is highly selective for a particular component will yield a large quantity of that component, as the permeate gas, downstream of the membrane. But as the permeate gas accumulates downstream of the membrane, the presence of such permeate gas impedes the further transport of that component through the membrane.
Various attempts have been made to overcome the above problem, such as by providing multiple membrane separation stages, and by the recycling of output streams. However, such solutions have often required more energy in moving the gases through the system. Module arrangements have been proposed which operate at lower pressure, so as to minimize power requirements, but such solutions have also limited the amount of gas which can be processed.
The present invention provides a method and apparatus which allows greater throughput, while providing product gas streams of high purity, and while still minimizing energy requirements.